Optical fibers are used to transmit data by transmitting electromagnetic radiation. The optical fibers are generally made of a glass or plastic core and enclosed in a cladding with a lower refractive index. The optical fibers transmit light through their length by internal reflections. The transmitted light can be modulated to transmit a signal, either analog or digital. Optical fibers can therefore be used to carry audio signals or data signals within a network.
Optical fibers are housed within a connector to mate one or more of the optical fibers to another optical fiber or set of optical fibers, or to a data-generating or data-using item of network gear. The connector may be formed of a ferrule which holds the optical fibers. The end face of the ferrule mates with the end face of a mating ferrule, to align each optical fiber with a corresponding optical fiber in the mating ferrule, to achieve transmission of signal through the optical fibers to the optical fibers of the mating ferrule.
The termination must protect against light loss, which is caused by a number of factors. In general, the more perfect the alignment between two optical fibers, the less light loss occurs at the connection. Light loss can arise from an air gap between two mating optical fibers or from a lack of alignment between two mating optical fibers. (Light loss can also arise from dirt on the optical fiber ends or from a rough surface on the optical fiber ends. These problems are generally addressed by cleaning the optical fibers during installation and by polishing the optical fibers.)
An air gap between two optical fibers causes Fresnel reflection, caused by the change in refractive index between the glass optical fiber and the air. The light reflected in this manner is lost in transmission. Improper alignment causes directional loss, when light propagating out of one optical fiber does not all enter the mating optical fiber and therefore does not transmit further. It is desirable to minimize the amount of light lost in transmission, whether by Fresnel reflection or by directional loss. As devices use more and more data, at faster and faster rates, the need to minimize the amount of light lost at an optical fiber termination becomes even more important.
An MT ferrule is a plastic member carrying multiple optical fibers, usually used for predetermined cable assemblies. The optical fibers are usually positioned in a plane. Two bores or guide holes in the ferrule are aligned parallel to the optical fibers and in the same plane. The bores or guide holes have a consistent dimension along the entire length thereof. Guide or alignment pins extend through the guide holes of the ferrule and into corresponding guide holes of the mating ferrule to keep the ferrule properly aligned. The guide or alignment pins are constructed to form an interference fit into the guide holes of the ferrule and to form an interference fit with the corresponding guide holes of the mating ferrule, to keep the ferrule securely linked to the mating ferrule and to keep each optical fiber aligned to its corresponding optical fiber in the mating ferrule. The secure link also protects against dirt, moisture, vibrations, shock, electromagnetic interference, and radio frequency interference.
End-face tilt is the angle formed by the imperfect alignment of the facial plane of the ferrule with the facial plane of the mating ferrule. The facial plane of the MT ferrule, perpendicular to the plane in which the optical fibers are arranged, is polished to minimize end-face tilt (as well as to clean the optical fibers and to smooth the surface of the ends of the optical fibers). Polishing is a difficult operation and some end-face tilt will always result, as it is difficult to achieve a perfectly flat facial plane by polishing. End-face tilt is illustrated in FIGS. 1a and 1b, which show two MT ferrules 2 and 4 mated in, respectively, the x axis (width) and the y axis (depth). The end faces 6 and 8 of the two ferrules 2 and 4 do not align perfectly, leaving a gap 10 in the x axis and 12 in the y axis, between the two end faces 6 and 8.
Optical transmission of light through the optical fibers of the MT ferrule is optimized when the end-face tilt is less than 0.03 degrees. Commercial polishers, however, can only achieve end-face tilt of 0.2 degrees. This amount of end-face tilt has a tremendous impact on ferrule performance, as it causes significant insertion losses. An off-square facial plane of an MT ferrule creates an air gap between the mating optical fibers, which leads to Fresnel reflection as described above. This lossy gap between two facial planes, caused by end-face tilt, causes loss of light, and therefore loss of data, at the interface of the MT ferrule and the mating ferrule. Higher end-face tilt causes increasingly higher insertion losses.
The guide or alignment pins must fit tightly in order to prevent offset of the lateral optical fiber-to-optical fiber alignment, and the tight fit of the guide or alignmentpins in the guide holes prohibits any rotation of the ferrule to accommodate end-face tilt.
Accordingly, a need exists for a way to connect an MT ferrule to another ferrule that minimizes the air gap caused by end-face tilt, but preserves alignment of the optical fibers.